A semiconductor wafer having semiconductor devices such as ICs and LSIs formed in the surface thereof is subjected to bottom face grinding to have a desired thickness. After that, the semiconductor wafer is divided into individual semiconductor chips along preliminary dividing lines called dicing lines (or streets), and the semiconductor chips are used as LEDs, MEMSs, or the like in various kinds of electronic apparatuses.
Examples of dicing methods of dividing semiconductor wafers into individual devices (chips) include the following methods:    1) A dicing method by which a semiconductor wafer is cut and divided along dicing lines with a cutting blade having thickness of approximately 20 to 40 μm, for example;    2) A dicing method using a scribing technique by which a diamond wheel having a shape of an abacas bead or a diamond blade is put into contact with a semiconductor wafer to scratch the surface of the semiconductor wafer like glass cutting, and the semiconductor wafer is then cleaved with a breaker;    3) A dicing method using abrasion by which laser energy is gathered into an extremely small area on a semiconductor wafer in a short period of time by using a laser beam such as YAG laser, so that the solid is sublimed; and    4) A dicing method by which a pulse laser beam having a wavelength that exhibits permeation properties for semiconductor wafers is emitted.
According to the method of 4), the light collecting point is set inside a semiconductor wafer, and a pulse laser beam having a wavelength that exhibits permeability properties for wafers is emitted along dicing lines. Altered layers are formed in series in the semiconductor wafer, and an external force is then applied along the dicing lines having strength lowered by the formation of the altered layers. In this manner, the semiconductor wafer is divided into individual chips (Japanese Patent No. 3,408,805, for example).
In the case of a semiconductor wafer having devices such as power transistors formed in a surface thereof, an electrode coated with a metal film that is made of gold, silver, titanium, or the like, and has a thickness of several millimeters is normally formed on the bottom surface of the semiconductor wafer. A sapphire substrate used for LEDs also has a metal film that is made of gold, aluminum, or the like, and has a thickness of several μm on a wafer bottom surface thereof. The metal film serves as a reflection film to increase luminance.
In a case where a metal film is formed on the bottom surface of a wafer as described above, one of the above dicing methods 1), 2), and 3) is used, for example. Alternatively, after an operation is performed according to the method 4), a metal film may be formed on the bottom surface of a semiconductor wafer (Japanese Patent Application Laid-Open No. 2009-200140, for example).
However, such a metal film has viscosity. Therefore, if a semiconductor wafer having a bottom surface coated with a metal film is cut by a cutting blade according to the method 1) or the like, burrs appear in the outer peripheries of the bottom surfaces of the divided semiconductor chips. Also, metal dust adheres to the cutting blade, and causes clogging. As a result, the service life of the cutting blade becomes shorter.
According to the method 2), the diamond wheel having a shape like an abacus bead or the diamond blade is expendable. For example, after two sapphire substrates of 2 inches in thickness are processed, the diamond wheel or the diamond blade becomes worn and needs to be replaced with a new one. The machine stoppage period required for the replacement leads to a decrease in throughput. Also, the replacement blade increases the operation costs, resulting in an increase in product price.
According to the method 3), a laser beam is gathered onto a metal surface, to melt the metal at the light collecting point, or turn the metal into a gaseous material or plasma. However, molten material or processing dust called debris is generated in either way. Therefore, the method is not suitable for semiconductor chip manufacture that dislikes processing dust.
According to the method 4), a pulse laser beam having a wavelength with permeation properties is used. However, a laser beam cannot permeate a metal film. Therefore, an altered layer cannot be formed in the substrate under the metal film. To counter this problem, there is a suggested method by which the metal film on the bottom surface is removed by etching, and dicing is performed by emitting a laser beam to the portions from which the metal film has been removed. In such a case, however, an expensive etching device is required, and the product price becomes higher accordingly.
The present invention has been made in view of the above circumstances, and the object thereof is to provide a dicing method that achieves excellent dicing properties at low costs by removing a metal film through a metal processing operation with a diamond tool and then performing pulse laser beam irradiation.